1. Field of the Invention
The present invention relates to an expansion turbine provided with a mechanism for braking using eddy currents.
Priority is claimed on Japanese Patent Application No. 2007-95447, filed Mar. 30, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Heretofore, in order to improve the thermal efficiency of refrigerating machines, expansion turbines have been used (for example, refer to Patent Document 1). As a method of braking the rotation of an expansion turbine for ultra low temperatures, there are known a hydraulic braking method and an electromagnetic braking method. As the hydraulic braking method, there are one using blower braking (for example, refer to Patent Document 2), and one using working fluid (for example, refer to Patent Document 3). As the electromagnetic braking method, one using a generator (for example, refer to Patent Document 4) is known.
Furthermore, as a method of braking the rotation of a rotating machinery in the vehicle field and the like, one using braking by eddy currents (for example, refer to Patent Document 5) is known. In this method, a principle is adapted whereby eddy currents are generated on the surface of the rotor by, supplying an electric current through a coil provided on the static side (stator) to generate a magnetic field, and inducing a magnetic field in the rotating rotor. The arrangement is such that an electromagnetic force is generated in the opposite direction to the direction of rotation by the vector product of these eddy currents and the magnetic flux, and it is used as a braking torque.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S60-228708    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H 6-137101    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H 8-310356    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2001-132410    [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H1-288636
Incidentally, since the method of blower braking among the above-described conventional expansion turbine braking methods is a mechanical structure, although the braking action is reliable, because it involves the flow of fluid, the flow of gas cannot be cut off completely. As a result, there is a concern about a drop in the turbine efficiency due to heat leakage to the blower side.
Furthermore, since a braking method using the generation of electrical energy involves high speed rotation, it requires an inverter for generation, and because the equipment for the braking resistance and for power regeneration when electrical energy is recovered and discharged by the inverter becomes correspondingly complicated, the control of the equipment also becomes complicated and expensive. Moreover, in the control method using the generation of electrical energy, there is a problem in that if the rotation conditions of a motor are not satisfied, the braking force is not exhibited.
Furthermore, as a braking method of rotating machines other than those in the above-described field of expansion turbines, for example a braking method using the mechanical friction of brake pads as used in the vehicle industry and the like is known. However, it is difficult to use this for the braking of a rotating body (rotor) that rotates at high speed as in an expansion turbine because there is a concern of disturbing the balance of the rotating body, for example by the heat generated due to friction loss, or losing balance due to contact.
Moreover, in the above-described conventional eddy current braking, since Joule heating is generated in the rotor due to the eddy currents, heat generation on the rotor side becomes prominent. In particular, in the case of high speed rotation via non-contact bearings, since there is no other method of removing heat than by gas or the like around the rotor, it is difficult to remove the heat on the rotor side. Furthermore, since the rotor is shaped such that it extends in the radial direction, such a shape is unsuitable for the braking of a high speed rotating body such as that in an expansion turbine. As a result, the development of an eddy current braking method has been required that has a simple, small, and inexpensive construction, in which a rotor can endure the centrifugal forces, and the rotating shaft stability is increased.
The present invention takes the above situation into consideration with an object of providing an expansion turbine in which heat generation on the rotor side can be suppressed, and also a braking torque can be obtained.